representation of damage. Broderick and Ahrens and van der Meer noted a zero- 

 damage level of 5 = 2. 



Thompson and Shuttler (1976) also used the eroded volume method. They 

 used a profiler with a foot diameter of D„5o/2 with soundings spaced at D„5o, which is 

 similar to that used in the WES experiments. The structure was surveyed after each 

 1,000 waves. They computed the eroded volume V^ using the trapezoidal rule and an 

 average profile. A damage number N^ was calculated, assuming a spherical armor 

 shape, as the number of stones eroded in a 9D„^q wide region of the breakwater section 

 as follows 



pV 





(3.3) 



which is equivalent to 



54 p! A^ 

 A^A = -f^ ^ (3.4) 



^Pa D 



over a 9D„5o width of the structure, where 

 p*^ = armor bulk density 

 V^ = average eroded volume 

 p^ = actual armor density 



The difference between the method of Thompson and Shuttler and the 

 damage index method is the 54p^Jnp„ in Equation 3.4. For Thompson and Shuttler's 



38 



